The extensive diversity of the mammalian T cell receptor (TCR) repertoire is generated by the somatic rearrangement of non-contiguous germline variable, diversity and junctional gene segments. Detailed knowledge of the extent and diversity of the TCR repertoire used in specific immune responses will facilitate our ability to understand the role of TCR genes in immune responses in normal and disease states. TCR spectratype provides a global survey of the expressed TCR repertoire by displaying the distributions of complementarity determining region-3 (CDR3) lengths for each of the TCR alpha and beta variable (V) gene families. Since the fine antigenic specificity of the TCR is determined primarily by the CDR3 region, analysis of this region can yield considerable insight into the molecular basis of a T cell response. Studies in the past year have been extended to include the TCRBV repertoire of macaques and the TCRAV repertoire of humans. Spectratype analyses of rhesus (Macaca mulatta) and pig-tailed macaques (Macaca nemestrina) was performed for TCRBV families expressed in total peripheral blood mononuclear cells (PBMC) and T cell subsets isolated from PBMC. These studies reveal that the T cell receptor repertoire of PBMC from healthy macaques often exhibits skewing in TCRBV family CDR3 profiles. The CD8+ T cell subset was responsible for the majority of observed skewing in CDR3 length profiles. However, CD4+ T cells were also shown to contribute to the skewed peripheral PBMC repertoire in these animals. An additional feature of the peripheral blood of the animals studied was the presence of an unusual population of extrathymic CD4+ and CD8+ (double-positive) T cells (up to 9.6% in the PBMC of rhesus macaques). The double-positive T cells could be differentiated from CD4 single-positive and CD8 single-positive T cells by their increased surface expression of LFA-1 and decreased CD62L expression. The percentage of the double-positive T cells was higher in rhesus than pig-tailed macaques and contributed substantially to the peripheral T cell repertoire. The human TCRAV repertoire was analyzed by spectratype analyses of whole PBMC and isolated T cell subsets including CD4+, CD8+, and CD4-CD8- (DN double negative) sub-populations. Skewing was observed primarily in CD8+ and in DN sub-populations and certain common features were observed among the fifteen healthy individuals studied. Expanded populations of cells with the same sized CDR3 length were present in TCRAV4A, AV7, AV19, and AV24 families in the DN and sometimes the CD8+ sub-populations. The expanded populations of cells with the same CDR3 length within each family were shown to correspond to a single or to a highly conserved sequence present in multiple individuals. These analyses suggest a central function for these conserved DN T cells present.